Mechanical Testing and Characterization of Proton Irradiated 99.4% NI Using the Reactor Materials Testing Laboratory
The Queen’s Reactor Material Testing Laboratory uses proton irradiation to simulate the damage induced in a material from neutrons, within a nuclear reactor environment. Inconel X-750 is a 70wt% Ni superalloy, used for the spacer material in the CANDU® reactor. In this work, 99.4% Ni is irradiated to 0.1dpa, with 6MeV protons, at an average temperature of 120oC using the Proton Irradiation Sample Holder. The motivation for this work was to distinguish between damage mechanisms versus phase transformation effects that are exhibited in X-750 from γ’ precipitates and disordered phases. Cross-sectional nano-indentation showed an increase in hardness with increasing dpa from proton irradiation. The Nix-Gao (NG) model, a strain gradient plasticity model, was applied to Indentation Size Effect (ISE) experimentation. A bi-linear trend was observed in the NG model for both the irradiated and unirradiated material. The increase in hardness, in the micro-scale regime, was measured to be 558.3±138.1MPa and using the Busby relationship the increase in the shear yield strength was measured to be 171.4±42.2MPa. TEM characterization identified three types of irradiation induced defects: 1/3<111> Frank Loops, ½<110> Perfect loops and SFTs, with a mean loop size of 7.20, 11.50 and 3.02nm, respectively. The total defect density was measured to be 3.1∙1022m-3, with 24.3% consisting of SFTs, 49.5% and 26.2% consisting of Frank Loops and Perfect Loops respectively. Using the hardening contributions of various microstructural features proposed by G.E Lucas, obstacle barrier hardening models and the super-position principle, the increase in shear tensile strength was calculated. The Bacon-Kocks-Scattergood model yielded a value of 169.8±25.8MPa, which directly agrees with the experimentally measured value. It is proposed that irradiation induced defects result in a smaller ISE, bi-linear transition at a shallower depth and a larger density of geometrically necessary dislocations (GND) to accommodate an indentation of specific depth, shallower than 500nm. The Ma and Clarke model and independent SEM/EBSD analysis provide evidence for a larger GND density in an irradiated material compared to an unirradiated material. It is believed that irradiation induced defects reduced the size of plastic deformation from an indentation and produced larger elastic recovery.